The proposed research concerns the design and application of novel, ultra-sensitive, highresolution laser spectroscopic methods in measurement of metallobiochemically-important elements. For dietary availability and mineral absorption studies, the use of stable isotopes has provided an attractive alternative to radioisotopes due to the safety and ethical reasons. Although stable isotopes have been used frequently, the currently available stable isotope determination methodologies limit severely the full use of stable isotopes in biotracer studies. Even the most often used method, mass spectrometry, has many disadvantages including slow sample throughput, chemical interference and requirement of time-consuming sample purification and chelation steps. In this proposal, innovative, powerful, laser-based methods - optical phase conjugation spectroscopy and polarization spectroscopy - are presented to overcome many of the difficulties. These Doppler-free (and Lorentzian-free) laser spectroscopic methods yield spectral resolution high enough to resolve atomic/isotopic hyperfine structures - the spectroscopic fingerprints of atoms. Hence, excellent elemental/isotopic selectivity is obtained and spectral and chemical interference is virtually eliminated even if the sample contains a number of minerals. In addition to selectivity, these nonlinear laser methods also offer one of the most sensitive detection limits, and, hence, they have potential for isotope analysis at trace concentrations (ppm, and pptr) or for simple quantitative measurement of minerals in biological samples. These nonlinear laser methods employ multiple input laser beams to induce nonlinear effects in the sample and to generate signal beams. Cubic signal dependence on laser power in optical phase conjugation spectroscopy (and quadratic dependence in polarization spectroscopy) allows extremely effective use of high photon power available from lasers. Since signal is a laser beam, optical detection is very efficient. While analytical flame can be used as an atomizer to provide fast, convenient and continuous-flow sample introduction, a low-pressure discharge cell can be used for small samples and to obtain extremely high spectral resolution. Some of the advantages of the proposed laser methods over the conventional methods include excellent sensitivity, selectivity and reproducibility, relatively inexpensive instrumentation, rapid, simple and safe operation, capability of analyzing both stable and radioisotopes in small sample sizes, and fast sample throughput. The effectiveness of these laser methods will be investigated for some of the most important elements in biological studies such as Li, Ca, Ba, Cu, Zn, Mg, Pb, Cd, V, and A1. Since all isotopes present (stable and radioactive) are measured simultaneously in a hyperfine structure, these laser methods are also applicable to studies where radioisotopes are absolutely necessary.